Non-curling gummed paper, method and composition for making same



NON-CURLING GUMMED PAPER, METHOD AND' COMPOSITION FOR MAKING SAME NoDrawing. Application January 11, 1954, 1

Serial No. 403,433

6 Claims. (Cl. 117-122) This invention relates to gummed paper, that is,paper having adherent thereto a film of dry adhesive, commonly referredto as gum, which when moistened with water, develops an adhesivetackiness permitting the application of the paper to another article ina position of use. Printed labels are articles consisting essentially ofpieces of such paper, and perhaps the most familiar example of such alabel is the ordinary postage stamp. Adhesive of the character referredto is commonly called a remoistenable adhesive. The thickness of thecoating film is usually specified in terms of adhesive to ream of paper.This is the equivalent of a statement of so many pounds to so many unitsof area. A fifteen to twentyfive pound coat of glue is common. Thecoating of dextrin on a postage stamp is thinner. Very roughly afifteen-pound coating is about a mil thick. Herein in referring to afilm I refer to a coating of the kind usual on gummed labels and similarproducts. The thickness is at most a few mils.

The object of the present invention is to provide such a gummed paperwhich will lie flat and will continue to lie flat under varyingconditions of atmospheric humidity to which the product may be exposed.

Gummed papers are commonly produced by preparing a solution of thechosen gum in a coatable consistency, applying a film thereof to paper,and evaporating the volatile ingredient to yield a dry, non-tacky,continuous film of potentially adhesive material.

Adhesive coated papers produced in this manner are troublesome inpractical use because of the tendency .of the gummed sheets or strips tocurl or roll up. This effect is due to the fact that the paper backingand the adhesive layer respond differently to changes in atmosphericmoisture content. Although both the paper backing and the adhesive layerexpand as they absorb moisture from the atmosphere and contract as theyrelease moisture to the atmosphere, normally this behavior is much morepronounced in the adhesive layer and thus it expands or contracts to agreater degree than the paper backing. Thus if such a sheet is subjectedto an atmosphere of high moisture content, the adhesive layer willexpand more than the paper backing, causing the sheet to curl away fromthe adhesive side. If such a sheet is subjected to an atmosphere of lowmoisture content, the adhesive layer will contract more than the paperbacking.

The most important and effective method resorted to by manufacturers inan attempt to counteract these disadvantages in producing what the traderefers to as gummed flats is the use of breakers. This involves passingthe coated sheet over a series of bars set at various angles causing theadhesive layer to be broken into small sections. Theoretically, eachsmall section thus produced reacts to variations in moisture content ofthe atmosphere independent of all other sections thus lessening thetendency of the'sheet to curl. In actual practice, this technique isonly partially .efiective since that portion of the adhesive layer incontact with the paper surface is reinforced by paper fibers and isparticularly resistant to rupture. Thus, though the surface UnitedStates Patent I ice of the adhesive layer is broken, that portion of thefilm in contact with the paper remains continuous and reacts tovariations of atmospheric moisture content with suflicient force tocause curl. Furthermore, if such sheets so processed are subjected to anatmosphere of high moisture content, each section of the adhesive layerwill expand and unite with adjacent areas tending to reform into acontinuous layer. For this reason, manufacturers of this type of productusually specify that finished material be stored in an atmosphere of lowmoisture content. The breaking is an added operation. It is hard tocontrol. Damage to the sheet may be caused and, because of the hightension used, rupture of the sheet is not uncommon.

In accordance with the present invention a paper is provided coated witha solid remoistenable film which to the naked eye (megascopically) iscontinuous and homogeneous, but it is composed in major proportion of anaggregation of minute discrete particles of solid but remoistenableadhesive which particles are adhered to the paper and to each other by aminor proportion of the film-forming adhesive which itself is watersoluble. Such a product may be prepared by mixing the primary adhseiveingredient in its minute particle form as a solid with a solution of thesecond or binder ingredient in a volatile liquid which does not dissolvethe primary ingredient, the resulting mixture being a fluent one ofcoatable consistency. This is coated on the paper in accordance with theusual practices of the coating art and the volatile liquid ingredientevaporated.

I have not attempted to illustrate by a drawing the nature of the film.Megascopically (to the naked eye) it appears homogeneous and continuous.A drawing would have to be on an enormously exaggerated scale and indetail arbitrary and diagrammatic. Thus, it might be misleading ratherthan informative. The individual discrete particles presumably differmarkedly in size and shape on the microscopic scale although in bulk andstatistiaclly they are a uniformly fine powder. Whether in a givenformulation the discrete particles which constitute the major proportionof the volume are immersed in a substantially continuous matrix ofmaterial cementing them to one another and to the paper, or whetherindividual particles are coated and thereby secured atpoints of contactto one another and to the paper is a question of microscopic orsubmicroscopic structure which might depend on the particularformulation. It may well be that both states might locally occursimultaneously, although from the point of view of the label maker orthe label user the applied film would be homogeneous and uniform.

It may be desirable at this point to illustrate what has been said ,bygiving an example and a counter-example. The following is an example ofthe invention:

Example A Material: Parts by weight Dextrin Polyvinyl methyl ether 20Toluol 45 .and consisting of separate finely divided particles ofdextrin bound together by the polyvinyl-methyl ether.

Patented May 28, 1957 ditioning, etc.) is required.

Material: Parts. by weight Dextrin 8O Polyvinyl methyl ether. 20 Water150 The dextrin and polyvinyl methyl ether are dissolved together inthewater, and agitated until a homogeneous solution is obtained. This makesa coatable composition which when applied to paper and dried byevaporation of the water, produces. a transparent homogeneous layer.

Unless theproduct of Example B is dried. slowly under controlledconditions of humidity and without mechanical restraint the sheet willform a, tube during drying and be impossible to handle. If mechanicallyrestrained until wound into rolls it will dry in an unbalanced conditionand, for example, a sheet severed from the roll will immediately curl.If conditioned, on breakers in such manner as to exhibit no curl at 50%relative humidity and 70 degrees F., an increase in atmospheric moisturecontent to 62% relative humidity or higher will cause a definite curlaway from the adhesive face. If the atmospheric humidity is reduced from50% relative humidity to 33% relative humidity or lower, a definite curltoward the glue surface results.

In the description of Example A above reference is made to the dextrinas being finely ground. At an earlier point I have expressed the idea byreferring to it as forming a. fine powder. Since, as I have stated, thefilm is megascopically homogeneous some idea of what is meant byfineness will be obvious. If the film coating is, for instance, to havean apparent thickness of only a mil, obviously, except for scatteredinstances, the particles must be less than a mil in mean diameter. Infact, preferably the great majority of them are much finer than that andtheir size could more suitably be expressed in terms of microns than interms of mils. The fineness is, of course, statistical. It is not to beassumed that they will be of identical sizes or shapes if accuratelymeasured in microns. It would of course be meaningless to say that theyare as fine as possible, but in general, the finer they are, short ofcolloidal size, the better the product. By minute I mean that the greatmajority of all the particles are smaller than the thickness. of thefilm. As an aggregation they are desirably as fine as it is commerciallyfeasible to produce them.

The use of dextrin as the solid ingredient is an advantageous choicebecause it is available initially in the form of a very fine powder,presumably reflecting the size of the starch granules from which it isproduced. In the case of bone glue or other potentially adhesivematerials not commercially available in the particle size desired, suchmaterial may be dry ground or ground in dispersed form in the solutionof the other component until a particle size is obtained which providesa satisfactory gummed product.

The particular materials given in Example A are not the only ones whichmight be used. For instance, the solid particles might be of ordinaryanimal glue instead of dextrin or of vinyl methyl ether-maleic anhydridecopolymer or of carboxy methyl cellulose. The binder ingredient might bevinyl methyl ether-maleic anhydride copolymer or hydroxy-propyl methylcellulose ether.

Suitable solvents, depending on the nature of the materials utilized asparticles and, as binder, may be chosen from in addition the binderingredient must be soluble in a solvent in which the primary minuteparticle ingredient is not soluble.

The proportions of the two may also be varied within considerablelimits, except that the solid particle component is the major proportionof the solid content of the film. Theoretically one might postulate thatthe solid particles should constitute the adhesive layer subject to therequirement that they should be essentially isolated one from anotherand secured by the secondary binding component and, conversely, that thelatter should be present in the minimum quantity necessary to secure theparticles together and to the paper. Assuming this to be an ideal, closeapproximationthereto is not required in practice, particularly when thebinder used is of a very flexible nature. Satisfactory practicalformulations may readily be determined by empirical methods.

It is apparent also that more than one material could be used indiscrete particle form and more than one material as the binderingredient provided the ingredients respectively satisfied the primaryconditions above referred to.

The results cited under Example A are characteristic of those obtainedby the practice of the invention. As they are empirically demonstrated Ideem it unnecessary to theorize as to the physical action which occurswhen humidity conditions to which the product is subjected vary,particularly since on account of the minute scale of the components ofthe film the action cannot conveniently be directly observed, althoughwhat takes place may be inferred with considerable confidence.

I am aware that the invention may be embodied in other specific formswithout departing from the spirit or essential attributes thereof, and Itherefore desire the present embodiment to be considered in all respectsas illustrative and not restrictive, as is in fact clear in severalmatters from the description itself. Reference is to be had to theappended claims to indicate those principles of the inventionexemplified by the particular embodiment described and which I desire tosecure by Letters Patent.

1 claim:

1'. Remoistenable gummed paper comprising a paper base having adherentto a face thereof a potentially adhesive solid fihn which when moistenedby water develops in the film an adhesive tackiness permitting thesecuring of the material in a position of use, the film comprising amultiplicity of minute, discrete, solid particles ofwater soluble gumintimately mechanically admixed as a dispersed phase in the film insubstantially uniform proportions throughout the mass of the film, andconstituting the major portion of its volume, the film also including asan external phase a different gum, likewise water soluble and alsosoluble in a solvent in which the first named gum is insoluble, coatingthe particles and cementing them together and to the base, the paper andthe adherent film being as a whole substantially flat and remaining sounder varying conditions of humidity to which they maybe exposed.

2. Remoistenable gummed paper as set forth in claim 1 wherein theparticles are of dextrin.

3. Remoistenable gummed paper as set forth in claim 1 wherein theparticles are of glue.

4. Remoistenable gnmmed paper as set forth in claim 1 wherein the othergum is polyvinyl methyl ether.

5. The method of making non-curling gununecl paper which comprisesdispersing a multiplicity of minute discrete solid particles ofwater-soluble gum in a solution, inlasolvent inwhich the aforesaid gumis insoluble, of

6 homogeneous and wherein the discrete particles as such form the majorportion of the volume.

References Cited in the file of this patent UNITED STATES PATENTS2,125,241 Humphner July 26, 1938 2,477,344 Neumann July 26, 1949 FOREIGNPATENTS France Jan. 5, 1945

1. REMOISTENABLE GUMMED PAPER COMPRISING A PAPER BASE HAVING ADHERENT TOA FACE THEREOF A POTENTIAL ADHESIVE SOLID FILM WHICH WHEN MOISTENED BYWATER DEVELOPS IN THE FILM AN ADHESIVE TACKINESS PERMITTING THE SECURINGOF THE MATERIAL IN A POSITION OF USE, THE FILM COMPRISING A MULTIPLICITYOF MINUTE,DISCRETE, SOLID PARTILCES OF WATER SOLUBLE GUM INTIMATELYMECHANICALLY ADMIXED AS A DISPERSED PHASE IN THE FILM IN SUBSTANTIALLYUNIFORM PROPORTIONS THROUGHOUT THE MASS OF THE FILM, AND CONSTITUTINGTHE MAJOR PORTION OF ITS VOLUME, THE FILM ALSO INCLUDING AS AN EXTERNALPHASE A DIFFERENT GUM, LIKEWISE WATER SOLUBLE AND ALSO SOLUBLE IN ASOLVENT IN WHICH THE FIRST NAMED GUM IS INSOLUBLE, COATING THE PARTICLESAND CEMENTING THEM TOGETHER AND TO THE BASE, THE PAPER AND THE ADHERENTFILM BEING AS A WHOLE SUBSTANTIALLY FLAT AND REMAINING SO UNDER VARYINGCONDITIONS OF HUMIDITY TO WHICH THEY MAY BE EXPOSED.